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Chapter 6 Acids and Bases Stomach Acid & Heartburn • the cells that line your stomach produce hydrochloric acid  to kill unwanted bacteria  to help break down food  to activate enzymes that break down food • if the stomach acid backs up into your esophagus, it irritates those tissues, resulting in heartburn  acid reflux  GERD = gastroesophageal reflux disease = chronic leaking of stomach acid into the esophagus 2 Curing Heartburn • mild cases of heartburn can be cured by neutralizing the acid in the esophagus swallowing saliva which contains bicarbonate ion taking antacids that contain hydroxide ions and/or carbonate ions 3 Properties of Acids • sour taste • react with “active” metals  i.e., Al, Zn, Fe, but not Cu, Ag, or Au 2 Al + 6 HCl AlCl3 + 3 H2  corrosive • react with carbonates, producing CO2  marble, baking soda, chalk, limestone CaCO3 + 2 HCl CaCl2 + CO2 + H2O • change color of vegetable dyes  blue litmus turns red • react with bases to form ionic salts 4 Common Acids Chemical Name Formula Uses Strength Nitric Acid HNO3 explosive, fertilizer, dye, glue Strong explosive, fertilizer, dye, glue, batteries metal cleaning, food prep, ore refining, stomach acid fertilizer, plastics & rubber, food preservation plastics & rubber, food preservation, Vinegar Sulfuric Acid H2SO4 Strong Hydrochloric Acid HCl Phosphoric Acid H3PO4 Acetic Acid HC2H3O2 Hydrofluoric Acid HF metal cleaning, glass etching Weak Carbonic Acid H2CO3 soda water Weak Boric Acid H3BO3 eye wash Weak Strong Moderate Weak 5 Structures of Acids • binary acids have acid hydrogens attached to a nonmetal atom HCl, HF 6 Structure of Acids • oxy acids have acid hydrogens attached to an oxygen atom H2SO4, HNO3 7 Structure of Acids • carboxylic acids have COOH group  HC2H3O2, H3C6H5O7 • only the first H in the formula is acidic  the H is on the COOH 8 Properties of Bases • also known as alkalis • taste bitter  alkaloids = plant product that is alkaline  often poisonous • solutions feel slippery • change color of vegetable dyes  different color than acid  red litmus turns blue • react with acids to form ionic salts  neutralization 9 Common Bases Chemical Name sodium hydroxide potassium hydroxide calcium hydroxide sodium bicarbonate magnesium hydroxide ammonium hydroxide Formula NaOH Common Name lye, caustic soda Uses soap, plastic, petrol refining soap, cotton, electroplating Strength Strong KOH caustic potash Strong Ca(OH)2 slaked lime cement Strong NaHCO3 baking soda cooking, antacid Weak Mg(OH)2 milk of magnesia antacid Weak NH4OH, {NH3(aq)} ammonia water detergent, fertilizer, explosives, fibers Weak 10 Structure of Bases • most ionic bases contain OH ions NaOH, Ca(OH)2 • some contain CO32- ions CaCO3 NaHCO3 • molecular bases contain structures that react with H+ mostly amine groups 11 Indicators • chemicals which change color depending on • the acidity/basicity many vegetable dyes are indicators anthocyanins • litmus from Spanish moss red in acid, blue in base • phenolphthalein found in laxatives red in base, colorless in acid 12 Arrhenius Theory • bases dissociate in water to produce OH- ions and cations ionic substances dissociate in water NaOH(aq) → Na+(aq) + OH–(aq) • acids ionize in water to produce H+ ions and anions  because molecular acids are not made of ions, they cannot dissociate  they must be pulled apart, or ionized, by the water HCl(aq) → H+(aq) + Cl–(aq)  in formula, ionizable H written in front HC2H3O2(aq) → H+(aq) + C2H3O2–(aq) 13 Arrhenius Theory HCl ionizes in water, producing H+ and Cl– ions NaOH dissociates in water, producing Na+ and OH– ions 14 Hydronium Ion • the H+ ions produced by the acid are so reactive they cannot exist in water  H+ ions are protons!! • instead, they react with a water molecule(s) to produce complex ions, mainly hydronium ion, H3O+ H+ + H2O  H3O+  there are also minor amounts of H+ with multiple water molecules, H(H2O)n+ 15 Arrhenius Acid-Base Reactions • the H+ from the acid combines with the OHfrom the base to make a molecule of H2O it is often helpful to think of H2O as H-OH • the cation from the base combines with the anion from the acid to make a salt acid + base → salt + water HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l) 16 Problems with Arrhenius Theory • does not explain why molecular substances, like NH3, dissolve in water to form basic solutions – even though they do not contain OH– ions • does not explain how some ionic compounds, like Na2CO3 or Na2O, dissolve in water to form basic solutions – even though they do not contain OH– ions • does not explain why molecular substances, like CO2, dissolve in water to form acidic solutions – even though they do not contain H+ ions • does not explain acid-base reactions that take place outside aqueous solution 17 Brønsted-Lowry Theory • in a Brønsted-Lowry Acid-Base reaction, an H+ is transferred  does not have to take place in aqueous solution  broader definition than Arrhenius • acid is H donor, base is H acceptor  base structure must contain an atom with an unshared pair of electrons • in an acid-base reaction, the acid molecule gives an H+ to the base molecule H–A + :B  :A– + H–B+ 18 Brønsted-Lowry Acids • Brønsted-Lowry acids are H+ donors any material that has H can potentially be a Brønsted-Lowry acid because of the molecular structure, often one H in the molecule is easier to transfer than others • HCl(aq) is acidic because HCl transfers an H+ to H2O, forming H3O+ ions water acts as base, accepting H+ HCl(aq) + H2O(l) → Cl–(aq) + H3O+(aq) acid base 19 Brønsted-Lowry Bases • Brønsted-Lowry bases are H+ acceptors any material that has atoms with lone pairs can potentially be a Brønsted-Lowry base because of the molecular structure, often one atom in the molecule is more willing to accept H + transfer than others • NH3(aq) is basic because NH3 accepts an H+ from H2O, forming OH–(aq) water acts as acid, donating H+ NH3(aq) + H2O(l)  NH4+(aq) + OH–(aq) base acid 20